1. Technical Field
The present invention relates to a gas sensor element for sensing the concentration of a specific component in a gas to be measured (to be simply referred to as a measurement gas hereinafter), a method of manufacturing the gas sensor element, and a gas sensor that employs the gas sensor element.
2. Description of Related Art
In exhaust systems of internal combustion engines of motor vehicles, there are generally provided gas sensors for sensing the concentration of a specific component (e.g., oxygen) in the exhaust gas (i.e., the measurement gas) from the engine.
Moreover, the gas sensors generally have a gas sensor element embedded therein. The gas sensor element has a main body that includes: a solid electrolyte body having oxygen ion conductivity and an opposite pair of first and second surfaces; a measurement electrode provided on the first surface of the solid electrolyte body so as to be exposed to the measurement gas; a reference electrode provided on the second surface of the solid electrolyte body so as to be exposed to a reference gas (e.g., air); and a porous diffusion-resistant layer through which the measurement gas is introduced to the measurement electrode.
The gas sensor element generally works with the solid electrolyte body heated to a high temperature (e.g., 500° C. or more) at which the solid electrolyte body can be activated. Therefore, when water drops included in the measurement gas come to adhere to the gas sensor element, a large thermal shock may be applied to the gas sensor element, thereby causing cracks to occur in the gas sensor element.
To solve the above problem, there is disclosed, for example in Japanese Patent Application Publications No. 2007-33374 and No. 2008-216241, a technique of covering the entire outer surface of the main body of the gas sensor element with a porous protective layer.
Specifically, according to the technique, the main body of the gas sensor element is first dipped in a slurry material for forming the protective layer. Consequently, the slurry material is applied on the entire outer surface of the main body of the gas sensor element. Then, the slurry material applied on the main body is dried and fired, thereby forming the protective layer.
However, with the above technique, it may be difficult to evenly apply the slurry material on the outer surface of the main body of the gas sensor element. More specifically, the main body of the gas sensor element is generally shaped so as to have a substantially rectangular cross section perpendicular to an axial direction (or a longitudinal direction) of the main body. Therefore, when the slurry material is applied on the outer surface of the main body by dipping, the thickness of the slurry material on the outer surface of the main body tends to be smaller at the corner portions of the main body than at the plane portions (or side surfaces) of the main body which extend between the corner portions. Consequently, the thickness of the resultant protective layer at the corner portions of the main body may be too small to reliably protect the main body from the water drops included in the measurement gas.
In addition, to secure a sufficiently large thickness of the protective layer at the corner portions of the main body, one may consider repeatedly applying the slurry material on the outer surface of the main body by dipping. However, in this case, the thickness of the protective layer is also increased at the plane portions of the main body, thereby increasing the heat capacity of the gas sensor element that includes the main body and the protective layer. Consequently, in operation of the gas sensor element, the time required for heating the solid electrolyte body to its activation temperature is accordingly increased, thereby making it difficult to ensure prompt activation of the solid electrolyte body.